The present invention is directed to a process for producing condensed stabilized hydrocarbons by flash pyrolysis of carbonaceous material.
Fluid fossil fuels, such as oil and natural gas are becoming scarce as these fuels are consumed by a world whose population is continually growing. As a consequence, considerable attention is being directed toward pyrolyzing solid carbonaceous materials such as coal to useful liquid and gaseous hydrocarbon products. Pyrolysis processes vary widely and include transport flash pyrolysis where pyrolysis occurs under turbulent flow conditions. A problem exists in maximizing the yield of liquid hydrocarbons having molecular weights useful for conversion to more valuable end products because of the presence of newly formed volatilized hydrocarbon free radicals in the volatilized pyrolytic vapor.
One of the first steps in the pyrolysis of carbonaceous material is the thermal generation of hydrocarbon free radicals via homolytic bond scission of the coal. These hydrocarbon free radicals will combine with each other to produce undesirable heavy molecules such as heavy viscous tars having high boiling points. These hydrocarbon free radicals will also combine with carbon sites, such as present on char, to form more char or coke.
A technique that has been used to upgrade tar liquids and improve middle distillate tar liquid yield, is the addition of gaseous hydrogen directly to the pyrolysis reactor. By hydrogenating volatilized hydrocarbons with gaseous hydrogen directly in the pyrolysis reaction zone, sulfur and nitrogen are removed as hydrogen sulfide and ammonia. Such hydrogenation directly in the pyrolysis zone also reduces the viscosity and lowers the average boiling point of the subsequently condensed volatilized hydrocarbons by terminating some hydrocarbon free radicals before they are allowed to polymerize to heavy tar liquids.
Processes involving such hydrogenation are disclosed in U.S. Pat. Nos. 4,162,959 and 4,166,786 both of which are incorporated herein by reference. These patents disclose a process wherein a carbonaceous material feed, hot heat supplying carbon-containing residue, and hydrogen gas are reacted in a transport flash pyrolysis reactor. Pyrolysis and hydrogenation of the pyrolysis products occur simultaneously.
The effectiveness of hydrogen gas in terminating hydrocarbon free radicals is directly related to the hydrogen partial pressure. The pyrolysis reactor is preferably operated at pressures slightly greater than ambient, although pressures up to about 10,000 psig may also be used. An increase in hydrogen partial pressure increases free radical termination. High pressures, however, increase both the capital and operational cost of pyrolysis. Therefore, the preferred hydropyrolysis pressure in the pyrolysis zone for economical operation is from about 1 psig to about 1000 psig.
Tar polymerization and cracking occur rapidly at pyrolysis temperatures. To minimize cracking pyrolysis vapors are rapid cooled and condensed by either direct or indirect heat exchange. Rapid cooling and condensation, although preventing some tar from cracking, are still not satisfactory in preventing a significant portion of the tar from polymerizing by free radical recombination in the liquid state.
A pyrolysis process is therefore needed which substantially eliminates undesirable volatilized hydrocarbon free radical reactions early in the formation of pyrolysis products, thereby increasing the yield of desirable lower molecular weight tar liquids having relatively low boiling points and decreasing the yield of undesirable heavy viscous tars having relatively high boiling points.